Circuits for charging heavy duty batteries are well known and are in wide spread use in industry. Batteries being charged are typically found in industrial trucks, communications equipment, or float charge rectifiers, and may have amp-hour ratings of from 200 amp-hours to 1800 amp-hours, more or less. Motive power batteries are usually re-charged overnight, a period which could range from eight to sixteen hours; communications batteries are continually on charge. Motive power batteries may need to be charged from a virtual zero charge to a full charge for use the following day, which is done by an initial high constant current at an appropriate voltage, followed by a constant voltage tapering current charge, followed by a trickle charge--all as taught in the aforementioned patent. Other charging procedures may simply be carried out first at a constant voltage, then at a constant current; or simply in a constant current float charging mode. In all events, the energy supplied to the battery is derived from an AC source through suitable rectification, and under suitable surveillance of the battery condition and control of a synchronous switch in the output to alter the rate of energy input to the circuit.
An example of one such circuit is the inventor's Canadian patent 1,111,104 issued Oct. 20, 1981, for a "Battery Charger and Surveillance System". All such circuits, in any event, derive the power that controls the input synchronous switch--that is, the control current through the control coil of the synchronous switch--from the DC output side of the circuit. They also derive the power for the sensing circuit that controls the control coil from the DC output side of the circuit; but the sensing power is a very low power requirement at all times, whereas the control coil power may, at times, be a very high power requirement.
It is therefore an object of the present invention to provide a circuit where the control power requirements are separated from the sensing power requirement.
Control of a constant current or tapered current charging circuit is often accomplished by a shunt control circuit, an example of which is discussed in the inventor's Canadian patent No. 822,798 issued on Sep. 9, 1969. Such a shunt control is used to control the amount of current flow through the control coil of a saturable reactor or other synchronous switch device having a control coil, such as a magnetic amplifier. The shunt generally consists of a transistor, the base of which is connected to the output of a sensing and control circuit.
When a substantially fully discharged battery is connected to the charging circuit and the charging circuit is turned on, the amount of current to initially charge the battery may be at a maximum, as determined by the status of the battery being sensed by the control circuit. Resultingly, the output from the control circuit causes current flow through the control coil as discussed hereafter, which in turn permits the saturable reactor gates to transfer a controlled amount of power to the transformer. Thus a controlled amount of DC charging current is available to the DC output for charging the battery. Usually, if the battery is substantially in a state of discharge, the current through the saturable reactor or other controlled input device will be at a maximum, and the DC output current will also be at a maximum.
When the battery becomes fully charged, the control circuit provides a signal to the shunt transistor, which is thereby fully turned on. As a consequence, very little, if any, current flows through the saturable reactor control coil. In turn, the saturable reactor gates permit very little, if any, power to be delivered to the transformer and ultimately to the DC output portion of the circuit.
A problem that has previously occurred, however, is one of reverse power flow from a partially or fully charged battery especially in the event of a failure of the AC input power. In this circumstance, reverse power flow from the battery travels back through the circuit to the control coil. It is highly undersireable to supply power from the battery back through the control coil, since the battery can become discharged during AC power failure. It is acceptable, however, to supply a very small amount of power to the sensing circuit.